I never progressed past 30km from hearing all of these people quiting to save their knees. I wondering where that sentiment comes from if there is no correlation.
My understanding is that people destroy their knees in many ways over the course of their lives and that running makes that painfully obvious to them so they stop.
I think its related to weight. We have to acknowledge that there is a serious obesity epidemic in the developed world. The majority of adults are overweight and obese.
This is anecdata from what I've seen: you see fat runners. you see old runners. You dont tend to see many fat old runners.
The other thing is that if you dont regularly change your running shoes, you will absolutely hurt yourself. I get knee pain if I don't swap out my shoes every 250 miles or 3 months.